Advances in integrated circuit design and manufacturing have enabled low cost, highly integrated, high performance image processing products, including the digital electronic cameras. A conventional camera comprises an image sensor, typically an array charge coupled device (CCD), an analog front end (AFE) and a digital image processor. The CCD is an integrated array of photocells used in digital imaging. Most analog front ends having optical black and offset calibration include schemes that integrate the error signal across a capacitor during an optical black period and feed back the voltage generated to the input to cancel the offset or the optical black value during the video interval.
As shown in circuit 100 of FIG. 1, a CCD (not shown) is connected to an AC coupling capacitor C, that clamps the direct current (DC) value of the input signal. An AFE connected to the capacitor C1 includes three main elements: a correlated double sampler 102 (CDS), a programmable gain amplifier 104 (PGA), and an analog to digital converter 106 (ADC). The signal output from PGA 104 feeds back into integrator 108. The feed back loop forms an optical black correction loop where the error signal is integrated and fed back to CDS 102. Integrator 108 couples to reverse programmable gain amplifier (RPGA) 110, the output of which feeds into CDS 102. As shown, the analog optical black level is sampled before it is digitized by ADC 106. Note, however, that the PGA 104 gain is in the optical black correction loop. RPGA 110 maintains the stability of the loop since its gain is inversely proportional to the PGA 104 gain. Since PGA 104 has gain range of one to fifty, RPGA 110 has an adjustable range corresponding to the reciprocal of the gain of PGA 104. If the circuit is implemented with switched capacitors, gain is achieved using the capacitor ratio. A wide gain range, however, requires either extremely large capacitor or extremely small capacitor.
Moreover, the loop gain of the correction circuit changes when the programmable gain changes. In order to keep the loop gain constant, the loop gain needs to be changed significantly because the programmable gain can change from 0 to 36 dB. In addition, this approach relies on device matchings which may cause a yield issue.
Thus, there exists a need for an analog optical black and offset correction circuit for CCD signal processing having a wide gain range that does not require a large capacitor or an extremely small capacitor.